$5106 and $5107 are used for the "4th nametable" that the MMC5 provides, which contains a single tile with a single attribute setting, filling the entire screen (very quick way of getting a "blank screen") - $5106 sets the tile number, and $5107 sets the attribute bits (bottom 2 bits). In my emulator, I simply do a memcpy() on each write, though I should eventually change this to use nametable I/O handlers (and store the MMC5 ExRAM within the mapper savestate instead of within the PPU as "the 3rd nametable").

--
Quietust
P.S. If you don't get this note, let me know and I'll write you another.